Answers

Much will depend on how nervous the mama cat is. If she's an experienced mother & she knows & trusts her human family, you can start gentle touching & handling when they're about a day old. But start slow & only a bit at a time.

It's good for kittens to get used to people early so they socialize well. But too much handling or by someone young or rough is worse than not handling them. It will only upset the mother or hurt the kittens. She may decide then to move them & her milk supply may suffer if she gets upset.

A finger stroked over the kittens the first while is enough. Later, when they start to move about more, one at a time, they can be gently picked up & kissed a bit. Keep an eye on mama. If she looks upset, put the kitten back & return to simply stroking them gently. Take your cue from her & how she reacts.

When you pick them up though, don't take them out of her sight but only handle them around her. As they become more mobile, you can increase the contact.

I agree with Morgana's mom. There really is no reason to constantly handle newborn kittens even though they're very cute. The only possible reason is if you notice one isn't getting fed enough and you need to supplement with KMR (Kitten Milk Replacement) or you need to help them potty by washing their bottoms with a warm washcloth. But if momma cat is doing a good job, it's a much better idea to look and not touch for a week or so. Even if momma is OK with her human family and not real nervous, it will not please her to have her babies handled constantly.